The present invention generally relates to an apparatus for estimating the body speed of a vehicle, and more particularly, to an estimated vehicle body speed calculating apparatus for calculating the estimated vehicle body speed in a position of each vehicle of a vehicle body to be reduced in speed.
During the reduced speed of the vehicle body, a vehicle wheel may be sometimes locked when a brake is normally applied and a brake is excessively applied. There is an antilock control apparatus which is adapted to release such locked condition. The antilock control apparatus is adapted to regulate brake fluid pressures so that a vehicle wheel speed may become a proper valve with respect to a vehicle body speed. This apparatus is provided to realize the stability of the vehicle body, improve the steering performance of the vehicle body during braking, and the shortening of the stopping distance.
It is necessary, first, to correctly obtain the vehicle body speed to be reduced in speed so as to improve the antilock control apparatus. Although a method of directly detecting the vehicle body speed by a sensor or the like as a means for obtaining the vehicle body speed, it is not practical in use, because it is extremely expensive. In many cases, a method of estimating the vehicle body speed from the vehicle wheel speed detected is adopted.
There are many known examples of methods of estimating the vehicle body speed. A typical method is to calculate an estimated vehicle body speed with the use of a maximum value from among the vehicle wheel speeds of four wheels.
This method causes internal and external wheel speed differences when the vehicle body turns and runs. As a result, the vehicle wheel on the internal wheel side becomes smaller than the estimated vehicle body speed obtained, thus causing an excessive pressure reducing operation of the brake fluid pressure. The stopping distance is extended by the excessive pressure reducing operation, which causes inconveniences in terms of safety running operation.
An apparatus for calculating the estimated vehicle body speed which is independent in right and left is provided (U.S. Pat. No. 4,439,832) so as to remove the inconveniences.
Since the true vehicle body speed in each vehicle wheel position during its turning and running operation is different not only in right, left, but also between the front, rear wheels, it is hard to say that correct estimated vehicle body speed is obtained in the respective vehicle wheel positions even if the right, left independent estimated vehicle body speeds are calculated as described hereinabove.
Although an apparatus for independently calculating the estimated vehicle body speeds in all the wheels is proposed (U.S. Pat. No. 4,675,819), correct controlling operation cannot be effected, because the friction coefficients .mu. on the road surfaces are not taken into consideration sufficiently in the apparatus.
In U.S. Pat. No. 4,675,819, the reduction speed of the estimated vehicle body speed is adapted to depend upon the road surface .mu. value, concretely to become proportional inversely to a vehicle wheel reduction speed d on the assumption that the vehicle wheel reduction speed d (absolute value) becomes smaller as the road surface .mu. value becomes larger, and that the vehicle wheel reduction speed d (absolute value) becomes larger as the road surface .mu. value becomes smaller when the vehicle wheel reduction speed has exceeded a given value, for example, it is -1.3 g or lower.
Actually the vehicle wheel behavior is disturbed by the control situation, the road surface variation, the dispersion. This method cannot estimate the correct road surface .mu. values. The estimated vehicle body speed is calculated by larger or smaller reduction speeds than necessary.
According to the apparatus, the estimated vehicle body speed is adapted to have the same value as the vehicle wheel speed before the vehicle wheel reduction speed exceeds the given value (-1.3 g). When the vehicle wheel reduction speed is a value larger than the given value, for example,-0.5 g, the estimated vehicle body speed is also large separated from the actual vehicle body speed so that the antilock control operation can be properly effected no longer.